Formaldehyde synthesis

ABSTRACT

A process is disclosed for the synthesis of formaldehyde from methane starting with the oxychlorination of methane to produce methylene chloride. Hydrolysis of methylene chloride yields the product formaldehyde. Byproduct chloroform and carbon tetrachloride are recovered and hydrogenated to provide additional methylene chloride.

FIELD OF THE INVENTION

This invention relates to a process for manufacturing formaldehyde frommethane. In the process, methane is first oxychlorinated with oxygen andhydrogen chloride to give chlorinated methane including methyl chloride,methylene chloride, chloroform and carbon tetrachloride. Methylenechloride is hydrolyzed with water to produce formaldehyde. The methylchloride is recycled to the oxychlorination step while the chloroformand carbon tetrachloride are treated with hydrogen to produce moremethylene chloride.

BACKGROUND OF THE INVENTION

Some formaldehyde is produced industrially by the oxidation of paraffinhydrocarbons, but the greatest amount is manufactured by the oxidationof methanol. When methanol is used as the raw material, two chemicalreactions may be employed. In one process, a catalyst is used for thevapor-phase oxidation of methanol. In the other process, a combinationof oxidation-dehydrogenation is used to convert methanol toformaldehyde.

Regardless of the process used to produce formaldehyde, certaindisadvantages are inherent in existing technologies. The oxidation ofhydrocarbons results in a complex mixture of products that are difficultto separate. Processes based on methanol are dependent on a raw materialthat fluctuates widely in availability and cost.

Therefore, it is an object of the present invention to provide for thesynthesis of formaldehyde starting with methane, which is the maincomponent of natural gas. The process should be both efficient androbust.

These and other objects, features and advantages of the presentinvention will become apparent from the following description taken withthe accompanying figures.

SUMMARY OF THE INVENTION

A process is provided for the synthesis of formaldehyde beginning withmethane. The process comprises separate reactions operated in concert.

In the first reaction, methane is oxychlorinated with oxygen andhydrogen chloride to produce chlorinated methane and water. The productsinclude methyl chloride, methylene chloride, chloroform and carbontetrachloride. This reaction is promoted by a catalyst.

The second reaction involves the reduction of chloroform and carbontetrachloride with hydrogen to yield methylene chloride and hydrogenchloride. Various catalysts may be used for this reaction.

Finally, the third reaction comprises the hydrolysis of methylenechloride with water to form formaldehyde and hydrogen chloride. Acatalyst is employed in this reaction.

As noted, the first reaction produces methyl chloride, which can berecycled to the oxychlorination reaction to produce additional methylenechloride. A stand-alone process is achieved by recycling hydrogenchloride produced in the second and third reactions to the firstreaction. In this manner there is no net consumption of chlorine values.

BRIEF SUMMARY OF THE DRAWINGS

The description herein makes reference to the accompanying drawingswherein like reference numerals refer to like parts throughout theseveral views and wherein:

FIG. 1 is a graph showing the conversions at equilibrium of methylenechloride to formaldehyde by undergoing hydrolysis with water.

FIG. 2 is a schematic flow sheet of the process showing the principalreactors and the auxiliary equipment.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT

Three separate reactions, operated in tandem, are employed to convertmethane to formaldehyde in high yields. By understanding the chemistryinvolved, a better appreciation of the merits of the process can beobtained.

The first reaction step, known as oxychlorination, achieves thechlorination of methane and recycled methyl chloride using as the sourceof chlorine. A catalyst is needed and can comprise a copper salt.Because of the poor reactivity of methane, a sufficiently high reactiontemperature is required, usually in excess of 375° C.

The distribution of products from the first reaction step can becontrolled only within broad ranges. Thus, although the object is toproduce methylene chloride, inevitably some methyl chloride, chloroformand lesser quantities of carbon tetrachloride will be produced. Inaddition, some methane usually escapes unreacted.

Depending on market conditions, there may be a demand for the byproductsmethyl chloride, chloroform and carbon tetrachloride. However, thissituation cannot be relied on, particularly for a large installation.Therefore, the recovery of chloroform and carbon tetrachloride iscritical to the success of the process.

Chloroform and carbon tetrachloride are recovered by reacting theseproducts with hydrogen in a second reaction step to give methylenechloride and hydrogen chloride. The equilibrium conditions for thesereactions are extremely favorable. Specifically, in the reduction ofchloroform with hydrogen to form methylene chloride and hydrogenchloride log equals 10.98 at 200° C. and has a value of 8.25 at 400° C.

Although hydrogenation can be conducted by a free radical reaction, acatalyst may be used to advantage. Prime candidates for catalysts arethe transition metals. These elements, in forming compounds, exhibitmultiple oxidation states. Among the transition elements of interest arecobalt, molybdenum, nickel, iron, and platinum In the third reactionstep, methylene chloride is hydrolyzed with water to produceformaldehyde and hydrogen chloride. As indicated by the data in FIG. 1,the yield of product is quite favorable. Thus, at 200° C. log K_(p) forthe reaction equals 3.40 while at 400° C., log K_(p) is 4.83.

Because methylene chloride is relatively stable, a catalyst is requiredfor the hydrolysis reaction. Several catalysts are usable, includingactivated carbon and tin phosphate. This list, however, is not meant tobe limiting. Conceivably, any catalyst used for the hydrolysis of methylchloride might be considered.

The process of the present invention can best be visualized by referringto the simplified flow sheet in FIG. 2. In this illustration, reactor 10is used for oxychlorination, reactor 20 for hydrogenation, and reactor30 for hydrolysis. Distillation columns 40 and 50 provide the separationand purification of the components in the streams shown. For the sake ofclarity, some details are omitted. For example, in the methyl chloriderecycle stream, some methane may be present. Also, the chloroform streamfrom distillation column 50 will probably contain carbon tetrachloride.

The present invention represents a comprehensive solution to themanufacture of formaldehyde from methane. The only required rawmaterials are methane, hydrogen and oxygen or air. There are nobyproducts or waste streams that must be handled. In this balancedoperation, maximum efficiency is attained. Finally, the chemistry isstraightforward and easy to scale up to a commercial unit.

1. A process for the synthesis of formaldehyde comprising the followingsteps operated in tandem: a. the reaction of methane and methyl chloridewith oxygen and hydrogen chloride in the presence of a catalyst toproduce methyl chloride, methylene chloride, chloroform, carbontetrachloride and water, b. the reaction of hydrogen with chloroform andcarbon tetrachloride to form methylene chloride and hydrogen chloride,and c. the reaction of water with methylene chloride in the presence ofa catalyst to form formaldehyde and hydrogen chloride.
 2. A processaccording to claim 1 in which the methyl chloride produced in the firststep is recycled to the first step.
 3. A process according to claim 1 inwhich the hydrogen chloride produced in the second and third steps isrecycled to the first step.